1. Field of the Invention
The present invention relates to an antifouling paint composition which is employed for preventing a marine structure or the like from damage caused by fouling aquatic organisms.
2. Description of the Background Art
A marine structure such as ship's bottoms, port installations, buoys, pipelines, bridges, submarine bases, aquaculture nets, fixed shore nets, installations related to submarine oil fields, conducting tubes or intake works of power stations, breakwaters or ship anchors are fouled by fouling aquatic organisms such as barna-cles, shellfish, bryozoan, algae and the like, leading to significant economic loss.
In order to prevent settlement of such fouling aquatic organisms, the marine structure is generally coated with an antifouling paint, which is mainly prepared from an antifoulant of an inorganic copper compound, an organic tin compound or the like. An antifouling paint contains rosin as a part of its paint binder, and the antifoulant is blended with the same, in order to prevent settlement of fouling aquatic organisms.
On the other hand, known is a hydrolytic antifouling paint which contains an antifouling component of a trialkyl tin polymer. This paint is so prepared that the trialkyl tin polymer is hydrolyzed in an underwater alkalescent atmosphere to elute an organic tin compound, while the paint binder is dissolved in water to elute the antifoulant as blended (refer to GB2084167, for example).
The antifoulant components eluted in such a manner have high toxicity and exhibit antifouling performance by killing or damaging the fouling aquatic organisms to be kept from settlement. Every one of the conventional antifouling paints contains a compound which is toxic for organisms including humans as hereinabove described, and hence the same is problematic in safety for operators. This is a serious problem in consideration of marine pollution, which has recently came into the spotlight.
In recent years, there have been proposed a number of silicone antifouling paints (refer to Japanese Patent Publication No. 63-2995 (1988), for example). Such a silicone antifouling paint utilizes water repellency and low surface free energy of a painted surface. However, the silicone antifouling paint has problems of imperfect adhesion to a coating substrate, insufficient strength of a paint film and difficulty in recoating and repair coating, although the same has low toxicity to the human body and a small degree of environmental pollution. Thus, the silicone antifouling paint is employed merely in a specific region of a nuclear power station, for example.
Under the present circumstances, it is extremely difficult to attain an antifouling function without employment of antifoulant. It is conceivably preferable to employ a hydrolytic binder, which can substantially reduce the amount of the antifoulant. In Japanese Patent Laying-Open Nos. 3-35065 (1991) and 62-13471 (1987), however, it is difficult to control hydrolyzability due to employment of an extremely hydrolyzable organosilyl group for serving as a hydrolytic group. In U.S. Pat. Nos. 5080892 and 4918147, the color tone is remarkably restricted since a binder itself contains a heavy metal while the same is a colored binder derived from metal ions.
On the other hand, biodegradable resin is recently watched with interest and various products employing the same have been proposed (refer to WO/92/21708, Japanese Patent Laying-Open No. 3-157450 (1991) and U.S. Pat. No. 5191037, for example). Biodegradation is conceivably caused by hydrolysis resulting from the function of external enzyme secreted by microorganisms.
An antifouling paint composition disclosed in Japanese Patent Laying-Open No. 4-120163 (1992) is known as a paint composition employing such a biodegradable resin binder. However, the biodegradable binder proposed in this gazette is not stably dissolved in a general purpose organic solvent for a paint such as xylene or toluene for a long time, and hence problematic in practical application. Further, the binder requires a specific synthesizing installation due to requirement for polycondensation reaction under decompressed and high-temperature conditions, as well as long-time reaction, leading to an economic problem to be solved.